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Last month, the Privacy Commissioner ruled that Hager's bank Westpac breached his privacy by releasing his personal records to the police without being compelled to do so.

Dirty Politics was released a month before the election and the allegations of dirty tricks by former Prime Minister John Key's office dominated much of the election campaign, though National's popularity was not dented and it eased to a third term in Government.

There is speculation that Hager's latest book could focus on Key, because the launch comes a day before the former Prime Minister gives his valedictory speech in Parliament.

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In an interview with the Spinoff in 2015, Hager said Key had "survived" the fallout from Dirty Politics despite "in many ways [being] at the centre of the distasteful politics".

"None of us knows really how politics works. He's survived at the moment by not answering the question and then not answering the question again and then refusing to answer the question again - relying on the lack of attention span of the media. He may get away with that, but I actually think that one is still playing itself out.

"I think that when people say John Key got away with the book, and never had to answer the question - and of course he has got away with not having to answer the question so far - I think they're not being optimistic enough. I think we may still see in the long run it will be seen to have bitten him badly and he hasn't got away with it."

Hager also wrote the The Hollow Men, which used leaked emails to reveal details about National's election strategy in the 2005 election campaign, and Seeds of Mistrust, which covered the alleged cover-up of a genetically-modified crop release in New Zealand.